1. Field of the Invention
The invention relates to a method for operating an internal combustion engine, in particular a diesel engine, and to an internal combustion engine suitable for implementing the method.
2. The Prior Art
The most important variables governing the combustion process in a combustion engine with internal combustion are the phase of the course of combustion, or rather the beginning of combustion, the maximum rate of increase of cylinder pressure, and the peak cylinder pressure.
In an internal combustion engine in which combustion occurs essentially by self-ignition of a directly injected volume of fuel, the governing variables are largely determined by injection timing, charge composition, and ignition lag. These parameters in turn depend on a multitude of variables, such as engine speed, amount of fuel, intake temperature, charge pressure, effective compression ratio, amount of inert gas in the cylinder charge, and temperature of the various parts of the engine.
Increasingly strict legal requirements necessitate the development of novel concepts in combustion design, in order to reduce the emission of particulates and NOx in diesel engines.
It is known that NOx and particulate emission in the exhaust gas may be reduced by increasing the ignition lag by advancing the start of injection, such that combustion occurs by self-ignition of a lean fuel-air mixture. A variant of this type is termed the HCLI-method (Homogeneous Charge Late Injection). In a combustion process of this type fuel injection takes place at a distance from top dead center of the compression phase that is large enough to give rise to a largely homogeneous fuel-air mixture. By means of exhaust gas recirculation the combustion temperature may be kept below the temperature required for NOx-formation. Since the homogenization of fuel and air is time-dependent, this method is limited as regards engine speed and load, and particle emission will increase if homogenization is insufficient.
U.S. Pat. No. 6,338,245 B1 describes a diesel engine with internal combustion operating according to the HCLI-method, in which combustion temperature and ignition lag are chosen in such a way that in the region of low to medium load the combustion temperature lies below the temperature of NOx-formation and the air ratio lies above the value at which particulates are produced. Combustion temperature is regulated by varying the exhaust gas recirculation rate, ignition lag is regulated via the timing of fuel injection. At medium to high load the combustion temperature is lowered such that NOx and particulate formation are both reduced. It is disadvantageous that especially in the medium load region a low air ratio combines with low combustion temperature and thus efficiency is lost.
U.S. Pat. No. 6,158,413 A describes a direct-injection diesel engine with internal combustion, in which-fuel injection does not occur before top dead center of compression and in which oxygen concentration in the combustion chamber is reduced by exhaust gas recirculation. This method is referred to as HPLI-method (Highly Premixed Late Injection). Because of the decrease in temperature level after top dead center—in comparison with conventional injection before top dead center—and the increased amount of recirculated exhaust gas—as compared to conventional operation—the ignition lag is greater than in the case of so-called diffusive combustion. The low temperature level regulated by exhaust gas recirculation causes the combustion temperature to remain below the value necessary for NOx-formation. The large ignition lag caused by retarded injection permits good mixing, thus avoiding local oxygen deficiency during combustion and in turn reducing the formation of particulate matter. The back-shifting of the combustion process causes a reduction of maximum temperature, but at the same time leads to an increase of mean temperature at a certain late crank angle, which results in an increased burning of particulates. Shifting the combustion into the expansion stroke in combination with a high exhaust gas recirculation rate will result in a rate of pressure increase in the cylinder which will remain within acceptable limits, despite a larger premixed fuel volume due to the large ignition lag and consequently a higher maximum combustion rate. A disadvantage of the method is again low efficiency at low load.
From the publication “Homogeneous Charge Compression Ignition (HCCI) of Diesel Fuel”, Allen W. Gray et al., SAE 971676, it is known that in combustion of a self-ignited lean fuel-air mixture very low emission values of NOx and soot are realised due to the homogeneous distribution of concentration and temperature. In the English-speaking world this method is known as the HCCI-method (Homogeneous Charge Compression Ignition).
It is further known to design pistons for diesel internal combustion engines with an essentially toroidal recess in the piston top, which is referred to as piston recess. In the transition area between piston top and piston recess a constriction or throat is provided resulting in a relatively narrow flow cross-section. The narrow flow cross-section will cause high mixture formation energy and thus substantially improved fuel conditioning. Pistons with such toroidal piston recesses are known from disclosures EP 0 383 001 A1, DE 1 122 325 AS, AT 380 311 B, DE 21 36 594 A1, DE 974 449 C or JP 60-206960 A, for instance. In conventionally operated internal combustion engines such pistons have the following beneficial consequences for the operating behaviour of the internal combustion engine: smoke-limiting full load may be increased; high compression ratios may be realised, resulting in lower combustion noise due to smaller ignition lag, less hydro-carbon emission, better start-up behaviour and an improved efficiency of the engine; there is also the possibility of retarding ignition without substantial increase of smoke, fuel consumption or HC-emission, since mixture formation energy remains high for a longer period of time. This possibility will above all lead to a decrease of nitrogen oxides, combustion noise and maximum cylinder pressure.
Furthermore, in DE 11 22 325 C1 a piston is described with a piston recess and a constriction, where a step-shaped depression is provided between squish surface and constriction.
In internal combustion engines operated according to the HCLI or the HPLI-method such piston types with a deep and constricted piston recess have not been used up to now, since it was assumed that start-up behaviour and thermodynamic efficiency would strongly deteriorate due to the deep piston recess and the strong squish flow. For this reason U.S. Pat. No. 6,158,413 A has proposed to suppress the squish flow completely by using a piston with a very shallow recess.
It is the object of the present invention to develop a method of operating an internal combustion engine, which will have minimum nitrogen oxide and particulate emission from low loads up to full load while maintaining high efficiency over the whole range.
According to the invention this object is achieved by the following steps:                operating the internal combustion engine in a first operating region corresponding to low partial loads, with essentially homogeneous mixture combustion and late fuel injection, starting fuel injection in a range of about 50° to 5° crank angle before top dead center of the compression phase;        operating the internal combustion engine in a second operating region corresponding to medium partial loads, with low-temperature mixture combustion and even later injection than in the first operating region, starting fuel injection in a range of about 2° crank angle before top dead center to about 20° crank angle, and preferably 10° crank angle, after top dead center of the compression phase,        with fuel being injected into the combustion chamber in the first operating region via first injection orifices and in the second operating region at least via second injection orifices of an injection valve configured preferably as a double needle nozzle.        
In the first operating region the internal combustion engine works according to the HCLI-method, in which fuel injection is timed relatively early in the compression stroke, i.e., in a range of approximately 50° to 5° crank angle before top dead center. Fuel injection in this first operating region occurs preferably at pressures between 400 and 1,000 bar. Combustion mainly occurs between 10° crank angle before and 10° crank angle after top dead center, resulting in very high efficiency. Due to the relatively high exhaust gas recirculation rate of between 50% and 70% the local combustion temperature lies below the temperature where NOx are produced. The local air ratio remains above the limit for particulate formation. Exhaust gas recirculation may be realised by external or internal recirculation or by a combination of both using variable valve control.
In the second operating region the internal combustion engine is operated according to the HPLI-method. Here the main part of the injection phase lies after top dead center of compression. Due to the lower temperature level after top dead center—as compared with conventional injection before top dead center—and the increased amount of recirculated exhaust gas of between 20% and 40% vis-a-vis conventional operation, the ignition lag is increased. If necessary, further measures may be adopted to increase the ignition lag, such as lowering the effective compression ratio and/or the intake temperature, or shortening the duration of injection by increasing injection pressure and/or increasing the cross-section of the injection nozzle orifices. The duration of injection is chosen such that the end of injection lies before the start of combustion. In this case emission of particulates may be kept at a very low level. This may be explained by the fact that the simultaneous occurrence of liquid fuel in the fuel jet and of the flame surrounding the jet in the conventional case is avoided, whereby oxidation reactions in the vicinity of the jet, which take place under conditions of air deficiency and will generate particulates, are also eliminated. For the combustion method of the second operating region injection pressures of at least 1,000 bar are required. The advantage of the method lies in very low NOx and particulate emission and in the relatively high exhaust gas temperature, which in turn is advantageous in the regeneration of devices for filtering the particle-exhaust gas stream.
Preferentially it is provided that in the first operating region fuel is injected at a lower rate of flow than in the second operating region. Particularly low nitrogen oxide and particulate emissions may be achieved if in the first and second operating regions fuel is injected in the shape of fuel jets forming a conical surface, with the apex angle of the cone in the first operating region differing from that in the second operating region, preferably by being smaller in the former.
In further development of the invention it is provided that in a third operating region corresponding to higher partial loads or to full load the main part of fuel injection takes place in the range of 10° crank angle before to 10° crank angle after top dead center, where it is preferably provided that in this third operating region multiple injection be used. The exhaust gas recirculation rate in this region amounts to as much as 30%, and preferably 10% to 20%. This will ensure good performance combined with low NOx and low particulate emission.
In the third operating region fuel may be injected through the first and/or through the second injection orifices.
The internal combustion engine is operated in the first, second and/or third operating region with an overall air ratio of approximately 1.0 to 2.0.
In a preferred embodiment it is provided that exhaust gas recirculation be carried out externally and/or internally and that swirl be variable for at least one region, and preferably for all three regions. Swirl values between 0 and 5 will lead to good exhaust gas values at low fuel consumption.
Furthermore it is of advantage if the geometric compression ratio is variable. The geometric compression ratio should be variable in the range from 13 to 19. A high compression ratio is advantageous for the coldstart phase. Reducing the compression ratio as the load increases will increase the maximum load attainable in the first as well as in the second operating region and will reduce particulate emission due to larger ignition lag.
In this context it may be provided that the effective compression ratio be varied by shifting the closing time of at least one intake valve. By delaying the closing of the intake valve or by a very early closing of the intake valve the effective compression ratio may be reduced, thereby permitting a reduction of the exhaust gas recirculation rate required for low NOx and particulate emission. It is possible to shift both the opening time and the closing time of the intake valve, or to shift only the closing time.
In a further variant of the invention it is provided that the changeover from first to second operating region, or back from second to first region, be initiated by reducing or increasing the exhaust gas recirculation rate. Alternatively, the changeover from first to second operating region of the engine or vice versa may be initiated by reducing the internal or external recirculation rate and by retarding the beginning of injection, respectively by increasing the recirculation rate and advancing the start of injection.
Preferably it is provided that the reduction of the exhaust gas recirculation rate on changing from first to second operating region of the engine be achieved by appropriately controlling the opening and/or closing time of the intake valve.
The effective mean pressure in the first operating region is preferably between 0 and 6 bar, and more preferably 5.5 bar, in the second operating region between 3.5 and 8 bar, and more preferably between 4 and 7 bar, and in the third operating region at least 5.5 bar, and more preferably at least about 6 bar.
For implementation of the method a direct injection diesel engine is required, with at least one cylinder with a reciprocating piston, in which the beginning of fuel injection may be varied at least between 50° crank angle before top dead center and 20° after top dead center, and preferably up to 50° after top dead center, and in which the exhaust gas recirculation rate may be varied between 0% and 70%. Furthermore it is provided that fuel injection pressure be variable at least between a first and a second pressure level, where the first pressure level preferably covers a range of 1,000 bar or less, and the second pressure level covers a range of 1,000 bar or more. Furthermore, a device for changing the swirl level may be provided.
It is also of advantage if the opening time and the closing time of the intake process are variable. To this end it is advantageous if the timing of the intake valve and also the timing of the exhaust valve may be shifted by means of a phase shifting device. It is of particular advantage if at least one intake valve can be activated during the exhaust phase.
Additionally or alternatively, activation of at least one exhaust valve during the intake phase may be provided.
Injection is best performed using a double needle nozzle with a first and a second injection orifice, which may be activated individually.
To obtain different rates of flow in the first and the second operating region it may be provided that the first injection orifices have a smaller total flow cross-section than the second injection orifices.
Since different combustion strategies are employed in the first and second operating region it is of advantage if the axes of the first injection orifices are situated along a first conical surface and the axes of the second injection orifices are situated along a second conical surface, where the apex angle of the first cone may be smaller than the apex angle of the second cone.
In a particularly preferred variant of the invention it is provided that the first and the second nozzle needle are coaxial, with the first needle preferably guided in the second needle, which is configured as a hollow needle. Alternatively it is also possible to position first and second needle in parallel, side by side in a nozzle holder.
Double needle nozzles with coaxial needles or needles positioned parallel to each other are known from DE 100 40 738 A1.
For HCLI-operation it is provided that at least one piston with at least one squish surface and a toroidal piston recess and a constriction in the transition area between squish surface and piston recess be provided, and that a squish flow directed from outside into the piston recess be generated during the upward stroke of the piston, and that fuel is at least to the greater part injected into the toroidal piston recess and transported by the squish flow along the side wall of the piston recess and/or along the piston bottom, evaporating at least partly along the way. The fuel jet is injected into the squish flow entering the piston recess. The squish flow transports the greater part of the fuel into the piston recess, where it evaporates and is approximately homogeneously mixed with the inflowing air. The flow in the piston recess depends on the presence or absence of swirl in the intake flow.
Preferentially it is provided that the fuel is injected in the direction of the constriction of the piston, with the axis of at least one fuel jet carrying the greater part of the fuel at the beginning of injection intersecting an area between the side wall of the piston recess and the squish surface, which area comprises an overhanging wall area, the constriction and an inflow area between squish surface and constriction.
In conventional diesel engines with internal combustion the intersection point of the fuel jet and the injection time are chosen in such a way that the fuel hits the overhanging wall area beneath the constriction at the beginning of injection—independently of the load situation. In the present invention it is provided that the intersection point be located in the overhanging wall area of the piston recess for low loads and that with increasing load the intersection point be shifted towards the constriction. This can be achieved by advancing the injection in time. Part of the fuel will thus be injected—against the squish flow—into the gap between piston and cylinder head. A great part of the fuel injected into the gap between piston and cylinder head will be washed into the piston recess by the squish flow. This will improve air distribution and mixture preparation and advantageously reduce HC and CO emission. Combustion of the fuel-air mixture occurs in the piston recess as well as in the gap between piston top and cylinder head.
Since the internal combustion engine is operated at relatively high exhaust gas recirculation rates of 50% to 70%, local combustion temperature lies below the temperature of NOx formation. The local air ratio remains above the value at which soot is formed. Exhaust gas recirculation may be of the internal or external type or may be achieved by a combination of both methods via variable valve control. Fuel injection occurs at an injection pressure between 500 and 2,500 bar. The main part of combustion occurs between 10° crank angle before and 10° crank angle after top dead center, resulting in a high degree of efficiency. The internal combustion engine is operated with an overall air ratio of approximately 1.0 to 2.0.
For HPLI-operation it is provided by the invention that at least one piston with at least one squish surface and a toroidal piston recess and a constriction in the transition area between squish surface and piston recess be provided, and that a squish flow directed from outside into the piston recess be generated during the upward stroke of the piston and a turbulent base flow be initiated in the piston recess, and that fuel be injected at least to the greater part into the toroidal piston recess and transported by the squish flow along the side wall of the piston recess and/or along the piston bottom, evaporating at least partly along the way. The flow in the piston recess depends on the presence or absence of swirl in the intake flow.
Combustion of the fuel-air mixture occurs in the piston recess as well as in the gap between piston top and cylinder head.
In HPLI-operation the main part of the injection phase lies after top dead center of compression. Because of the lower temperature level after top dead center—as compared to conventional injection before top dead center—and because of the increased volume of recirculated exhaust gas, between 20% and 40%—again as compared to conventional operation—, the ignition lag is increased. Further measures may be adopted to increase ignition lag, such as decreasing the effective compression ratio and/or the intake temperature, or shortening the duration of injection by increasing injection pressure and/or the cross-section of the injection nozzle orifices. Duration of injection is chosen such that the end of injection precedes the start of combustion. In this case particulate emission may be kept at a very low level. This can be explained by the fact that the simultaneous occurrence of liquid fuel in the fuel jet and of the flame conventionally surrounding the jet is avoided, whereby oxidation reactions in the vicinity of the jet, which are taking place under lack of air and are thus generating particulates, also are eliminated. The HPLI combustion regime requires injection pressures of at least 500 bar. The advantage of this regime lies in very low NOx and particulate emission and in a relatively high exhaust gas temperature, which in turn is advantageous for the regeneration of particle-exhaust-gas filtering devices. The internal combustion engine is operated with an overall air ratio of approximately 1.0 to 2.0.
It is also of advantage if the geometric compression ratio is variable. The geometric compression ratio may be varied over a range from 14 to 18. A high compression ratio is advantageous for the cold-start phase. Reducing the compression ratio as load increases will result in a greater attainable maximum load and will decrease particulate emission due to a greater ignition lag.
In this context it may be provided that the effective compression ratio be varied by shifting the closing time of at least one intake valve. By delaying the closing of the intake valve or by a very early closing of the intake valve—roughly 0° to 20° crank angle—the effective compression ratio may be reduced, thereby permitting a reduction of the exhaust gas recirculation rate required for low NOx and particulate emission. It is possible to shift both the opening time and the closing time of the intake valve, or to shift only the closing time. By shifting the end of intake to “late” in at least one operating region combustion noise may be significantly reduced. Smoke formation may be reduced and fuel consumption especially at higher loads may also be reduced. By shifting the end of intake forwards or backwards compression end temperature can be lowered and ignition lag can be increased. If the intake end is shifted to “early” there is an additional effect of charge cooling. Via the closing time of the intake valve the course of combustion, in particular the position of the main part of combustion (MFB 50%—mass fraction burned) can be controlled. This may be done additionally or in combination with MFB-50%-control via injection timing or as stand-alone control action in the case of nearly homogeneous HCCI-combustion (Homogeneous Charge Compression Ignition), where no correlation exists between start of injection and start of combustion or MFB-50%-time. Dampening of combustion, the curve of the heat release rate or position and value of the maximum gradient of cylinder pressure dp/dα (p . . . cylinder pressure, α . . . crank angle) may also be controlled via the closing time of the intake valves, either additionally to or combined with dp/dα-control by internal exhaust gas recirculation, or as a stand-alone control action in the case of nearly homogeneous HCCI-combustion, where no correlation exists between injection start and start of combustion or MFB-50%-time. Furthermore the ignition lag (i.e. the time-window between injection start and start of main combustion without prior reactions) may be optimally controlled via the closing time of the intake valves, in particular combined with internal or external exhaust gas recirculation.
In HCLI- as well as in HPLI-operation it is of advantage if an intake swirl flow with swirl value ≧1 is generated in the cylinder and if the fuel is transported by the squish flow downwards along the wall of the piston recess, where it evaporates at least partly, and subsequently along the bottom towards the center of the recess. The swirl is kept in being within the piston recess during the compression phase.
In another variant it is provided that a swirl-free intake flow with a swirl value <1 is generated in the cylinder and that the fuel is transported by the squish flow from the center of the recess along the bottom to the wall of the piston recess and farther on towards the constriction, evaporating at least partly along the way.
Somewhat surprisingly it has been found that the depressed piston recess does not significantly impair the start-up capability of an internal combustion engine operating under the HCLI and/or the HPLI-regime. The decrease of thermodynamic efficiency due to the squish flow will be more than compensated by improved mixture preparation in the piston recess as a consequence of high turbulence.
For implementation of the method an internal combustion engine with at least one injection device for direct fuel injection is suitable, which is also provided with a device for exhaust gas recirculation and with at least one cylinder with a reciprocating piston, which piston is provided with a clearly defined squish surface and a toroidal piston recess. In the transition area between squish surface and piston recess the piston has a circular constriction. This will on the one hand result in a significant squish flow and on the other hand will ensure that the flow enters the piston recess with relatively high velocity. The relatively high level of turbulence inside the piston recess is beneficial for thorough combustion and will result in significantly reduced HC and CO emission. It is of particular advantage if the piston recess is dimensioned such that the ratio of maximum recess diameter DB to piston diameter D lies in the range 0.5<DB/D<0.7 and the ratio of maximum recess depth HB to piston diameter D lies in the range 0.12<HB/D<0.22. This will permit the free length of the fuel jet to be as long as possible. For the formation of a marked squish it is preferably provided that the piston recess be dimensioned such that the ratio of the diameter of the constriction DT to maximum recess diameter DB lies in the range 0.7<DT/DB<0.95.
Between squish surface and constriction an inflow region is provided in the shape of an annular depression with plane bottom and cylindrical wall. Preferentially it is provided that the depth of the depression is between 5% and 15% of the maximum recess depth, that the depression has an at least partly cylindrical wall and that the diameter of the depression in the region of the wall is 10% to 20% larger than the diameter of the constriction. The depression will cause a decrease of the radial velocity of flow out of the recess during the downward stroke of the piston. Part of the fuel is thereby guided in axial direction towards the cylinder head instead of along the piston top.
In order to increase cylinder charge especially in the case of high exhaust gas recirculation rates and to extend the range of loads in the HCLI-region, it may be provided that the internal combustion engine be operated in at least one operating region with pulsed supercharging. An internal combustion engine and a method for pulsed supercharging are for instance known from DE 199 08 435 A1.
In order to avoid NOx peaks at the end of short deceleration phases it is advantageous if during deceleration or overrun a throttle flap positioned in the inlet passage is closed and an exhaust gas recirculation valve in the exhaust gas recirculation duct is opened. From DE 101 18 878 A1 it is known to recirculate exhaust gas during deceleration of the internal combustion engine in order to avoid a temperature drop in the catalytic converter during prolonged phases of deceleration.
To avoid high particulate emission as a consequence of a fuel-air ratio that is too low it may be provided that the maximum permitted injection volume is computed from the minimum permitted fuel-air ratio and the actually measured mass of fresh air. Alternatively to measuring the mass of fresh air the present fuel-air ratio may be measured by a probe.
The actual present value of at least one combustion parameter used for combustion control may be computed as a weighted mean of the values in previous individual cycles. The actual present value of the position MFB50ist of the main part of combustion for a cycle z may be computed by the following formula, using weights of 40%, 30%, 20%, 10% for the last four individual cycles:MFB50ist(z)=0.4·MFB50(z−1)+0.3·MFB50(z−2)+0.2·MFB50(z−3)+0.1·MFB50(z−4)
Furthermore it is of advantage to adapt the parameters of a combustion controller (e.g. a MFB50 controller) as a function of the target value. The parameters of the controller are adapted depending on the target value. If the position of the main part of combustion MFB 50% shifts in the late direction (e.g. in HPLI-operation) the influence of injection timing on the position of MFB50 increases (system gain increases). Therefore the control parameters (amplification factor) are adapted as a function of the target value MFB 50% in such a way that the transfer function of the closed control loop does not change for late combustion.